Take lessons. I was a competent (but not good) swimmer, I even took a swimming course in college (where I became good, but forgot it after years of not swimming). At 30 or so I took a 4 week course with an instructor, and had 1-on-1 training (mostly, she had two true beginners there as well). I swam laps working on specific details while she worked with them, I got dedicated attention just before they arrived and after they left.

There were a lot of things about the mechanics of swimming I "knew", but had not internalized (as evidenced by my form and approach to the water). It was a good experience for me, and I ended up swimming for exercise for about 6 months after (shoulder injury took me out of it and I lost access to that pool).